It is well known in the art that the reaction transmitted from the steered road wheels of an automotive vehicle to the driver's manipulative effort applied to the steering wheel when the road wheels are being steered increases with a decrease in the vehicle speed and peaks up when the vehicle is held at a halt. An up-to-date automotive vehicle is therefore equipped with a power-assisted steering system principally with a view to permitting a vehicle driver to maneuver the steering system with a reduced manipulative effort. A power-assisted steering system developed for this purpose has a steering pressure control valve in which the fluid pressure developed by a fluid feed pump usually driven by an internal combustion engine is regulated to vary with the driver's manual effort applied to the steering wheel. The fluid pressure thus regulated by the steering pressure control valve is supplied to a steering power cylinder assembly so as to create a hydraulic steering assistance competent with the driver's manual effort applied to the steering wheel. The fluid feed pump of a power-assisted steering system is usually driven to deliver pressurized fluid not only when the steering system is maintained inoperative with the steering wheel held in the straight-ahead angular position but during left-turn and right-turn conditions of the steering system.
The steering pressure control valve thus used in a power-assisted steering system has incorporated therein a flow restriction which is causative of a resistance to the flow of fluid through the control valve. The resistance gives rise to an increase in the burden imposed on the fluid feed pump and invites a loss in the power produced by the internal combustion engine driving the pump under not only the inoperative condition but also the operative conditions of the steering system. The loss in the power delivered from the engine results in waste of fuel and accordingly in deterioration in the fuel economy of the engine. When, furthermore, a pressurized fluid is passed through the flow restriction in the steering pressure control valve at a high velocity, a vacuum tends to be developed immediately downstream of the flow restriction and to cause cavitation in the flow of the fluid through the flow restriction. The cavitation thus caused in the control valve in turn causes production of unpleasant noises and vibrations in the valve. To preclude the occurrence of cavitation in the control valve, it has been proposed to have an orifice provided downstream of the flow restriction so as to increase the fluid pressure at the outlet of the flow restriction so as to avoid development of the vacuum which is responsible for the cavitation. Provision of the orifice downstream of the flow restriction however results in addition of a resistance to the flow of fluid through the control valve and accordingly in an increase in the burden exercised on the fluid feed pump.
An advanced version of power-assisted steering has therefore been proposed in, for example, Japanese Patent Publication No. 51-42820 for providing a solution to these problems encountered in a prior-art power-assisted steering system of the described type. The steering system therein taught has a fluid-flow control valve provided in combination with a steering pressure control valve. The fluid-flow control valve is disposed in a fluid line leading from the steering pressure control valve to the suction port of the fluid feed pump and includes a fluid discharge passageway forming part of the fluid line, a valve chamber crossing the fluid discharge passageway, and a valve spool movable in the valve chamber. The valve spool is responsive to a differential fluid pressure developed between the pressure acting chambers of the steering power cylinder assembly and is formed with a circumferential groove which is communicating with the fluid discharge passageway depending upon the axial position of the valve spool in the valve chamber. In the presence of a differential fluid pressure acting on the valve spool, the circumferential groove in the valve spool constitutes a flow restriction in the fluid discharge passageway so as to increase the fluid pressure downstream of the flow restriction in the steering pressure control valve. In the absence of a differential fluid pressure acting on the valve spool, the groove in the valve spool provides unrestricted fluid communication through the valve chamber so that the fluid passageway has practically no flow restriction formed therein. A drawback inherent in a prior-art power-assisted steering system is, however, still encountered in such an advanced version of the steering system in that the steering pressure control valve per se has a flow restriction provided therein and, for this reason, can not eliminate the resistance imposed on the flow of fluid therethrough particularly under the inoperative condition of the steering system.
The present invention contemplates provision of an improved fluid-operated control apparatus for a steering system overcoming these drawbacks of prior-art power-assisted steering systems of the described general natures.